Wow, I can't believe the amount of people on here suggesting you keep it. Don't. Take it to the nearest police station to where you found it and give it to them. If no one comes in looking for it it will end up with you. Then you will have it with a clean conscience.
No it will not! The police will have forgotten about it or claim never to have known about it. Best thing to do was have the loser contact the local police and file a lost cash claim and they in turn could contact you with the info on the loser. That way your privacy is protected and you walk away with cash and a clear conscience........